Refactored 'sks' role

- servers in 'sks' role are now divided into frontends and backends;

- sks frontends are nginx servers configured as reverse proxy to sks
  backends. There is one frontend configured by default,
  'keyserver.your.domain'. SKS frontends do not run sks service by
  themselves, and if you want, you can disable the default keyserver and
  configure your own on a separate host. List of sks backends will be
  taken from 'sks_backends' list in the inventory;

- sks frontends copy default 'index.html' page to the root of the
  webserver, but only if it's not present, that way you can later supply
  your own and it won't be rewritten by Ansible;

- sks backends are servers running sks service. At least one of them
  should be able to connect to the SKS Keyserver public network if you
  want to share keys, but rest of them can be behind the firewall;